* glibc detected ./load: double free or corruption (!prev): ADDRESS **
When using glibc, how does it know that I am double-freeing? Does it keep track of everything I malloced and freed? Is it contained in metadata like how free knows how much space to free (How does free know how much to free?)
For each allocation, memory manager keeps some 'header' (most likely tree node or linked list). When you passed to free something that doesn't contain valid header - well, it couldn't correctly be freed. As for where this information is being kept - it's up to implementation, but usually it placed right before address you got from malloc - however, size and structure is very likely to be unknown, but at least it gives an idea how easily this header could be broken/corrupted/overwritten/etc.